It has long been known that only a portion of the oil can be recovered from an oil-bearing subterranean formation as a result of the natural energy of the reservoir. So-called secondary recovery techniques are used to force more oil out of the reservoir, the simplest method of which is by direct replacement with another medium, usually water or gas.
Water-flooding is one of the most successful and extensively used secondary recovery methods. Water is injected, under pressure, into reservoir rocks via injection wells, driving the oil through the rock towards production wells. The water used in water-flooding is generally saline water from a natural source such as seawater (hereinafter “source water”).
The factors that control crude oil/brine/rock interactions and their effect on wettability and oil recovery involve complex and sometimes competing mechanisms. It has been reported that oil recovery can be dependent on injection brine concentration. In particular, it has been shown in laboratory core studies by Morrow and co-workers that the use of a lower salinity injection water during water-flooding can increase oil recovery compared to the use of higher salinity water. SPE93903 describes later work with lower salinity water-flooding.
But lower salinity waters are often not available at a well site and would have to be made by reducing the total ion concentration of higher salinity water using techniques such as reverse osmosis or forward osmosis.
There is thus a problem of how to enhance recovery of oil from an oil-bearing formation using a method which is either cheaper for the same recovery or which gives better oil recovery for the same cost.